This invention relates to coated polypropylene film. More particularly, this invention relates to oriented polypropylene film coated with a submicron thickness of a composition comprising nitrocellulose, to its process of preparation and its use in the preparation of printing plates.
In the past, polymer film, particularly uncoated polypropylene film, has been used to separate the negative from the liquid photopolymer when preparing plastic printing plates. In such use, a negative, usually consisting of halftones and line copy, is covered with a thin transparent polymer film (sometimes called a cover film). Over the film is spread a liquid photopolymer and then a backing sheet or substrate is usually added. The cover film serves to protect the negative from the liquid photopolymer. This assembly is exposed through the negative to some form of radiation, e.g., actinic light, for a period of time sufficient to harden the liquid photopolymer in those areas where it is not shielded by the negative. The resulting exposed plate is then separated from the negative and the thin polymer film used to separate the negative from the photopolymer is peeled off the face of the resulting plate. Finally, the unhardened photopolymer is removed, usually by washing, from the unexposed areas and, if desired, the plate is hardened by post-exposure. While uncoated polymer films have been used as separators or cover films in processes as described above, they have caused certain difficulties. For example, the uncoated polypropylene film sometimes sticks to the plate and slivers or tears as it is peeled from it. Also, distortions in the image may occur which take the form of elongated highlight dots and broadened or split rules, which is attributed to photopolymer flow at the interface between the plate relief surface and the uncoated polypropylene film during the imaging (i.e. exposing) step.
U.S. Pat. No. 4,247,576 (Kunter) the disclosures of which are incorporated herein by reference in their entirety, discloses a silicate coated polypropylene film which was used for purposes similar to the purposes of this invention. However, when using waterborne coatings the silicate based materials of Kunter frequently cause streaking problems on the polypropylene film. When a streaky polypropylene film is used in an imaging process, such as a substrate or cover for imaging printing plates, the streaks cause imperfections in the imaging process.
This invention addresses the streaking problems associated with the compositions of Kunter and proposes a composition comprising nitrocellulose for coating the polypropylene film. These nitrocellulose coatings solve the streaking problem of Kunter, yet maintain the desirable release and other properties achieved by the Kunter silicate compositions.